4th step
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classifying collisions - number of charged tracks from collision
If any beam particle does NOT continue parallel through the picture, it must have collided with a proton. (Very occasionally a beam particle may "decay" )
The first quantity of interest is the number of charged particles coming
from the collision. Since we are studying the collisions of 
particles with the protons
the total charge before any collision is zero.
From charge conservation - one of the most important rules of particle physics - there must therefore always be an equal number of positive and negative particles after collision. Every charged particle that leaves a track in a bubble chamber has a charge of +/- the charge of the electron. (The possibility of neutral particles being produced will be discussed in the next section.)
Check charge conservation for our collision. You will see that there
are two positive tracks (curving to the left), one negative track (curving
to the right) and one straight track that kinks suddenly (move cursor
over the picture to see ), before travelling far enough to see which way it was
curving. Since the total final charge is zero, this kinking track must
be negative.
For an excercise click here